Project Summary Myeloid cell leukemia-1 (MCL-1) is an anti-apoptotic member of the BCL-2 family that is highly expressed in the myocardium. My lab recently discovered that cardiac-specific ablation of MCL-1 leads to mitochondrial dysfunction, rapid development of heart failure, and early mortality in the absence of any other stress. Surprisingly, MCL-1 deficient myocytes display signs of necrosis, rather than apoptosis as would be expected. This suggests that besides its anti-apoptotic role, MCL-1 has an essential but yet unidentified role in maintaining mitochondrial function in cardiac myocytes. MCL-1 exists both in the outer mitochondrial membrane (MCL-1OM) and in the mitochondrial matrix (MCL-1Matrix) in the heart. Since the cardiac-specific MCL-1 knockout mice are missing both forms of MCL-1, their specific contributions to mitochondrial function are unknown. Based on my preliminary data, I will explore the hypothesis that, in addition to sequestering pro- apoptotic proteins, MCL-1OM protects against cell death by facilitating Drp-1-mediated mitochondrial fission and removal of aberrant mitochondria via autophagy. This hypothesis will be tested with two specific aims. In Aim 1, I will delineate the role of MCL-1OM in regulating mitochondrial fission and mitophagy. I will explore whether MCL-1OM protects against cell death by facilitating Drp-1-mediated mitochondrial fission and removal of aberrant mitochondria via autophagy. I will also determine whether MCL-1OM promotes clearance by acting as a mitophagy receptor by interacting with LC3 on the autophagosome. In Aim 2, I will evaluate the cardioprotective function of MCL-1OM in vivo under baseline conditions and in response to myocardial infarction. These studies will provide important new insights into the relationship between mitochondrial dynamics, turnover, and survival in the heart. A better understanding of how mitochondrial function is regulated in the heart under normal and disease conditions such as myocardial infarct will contribute towards future clinical management of heart disease.